District Court Judge Richard Leon said the program appeared to violate the Fourth Amendment prohibition on unreasonable searches. He added that the Justice Department had failed to show the bulk collection of telephone records had helped prevent a terrorist attack.
In his written ruling, a copy of which was published by the Washington Post, Leon said a lawsuit filed by conservative legal activist Larry Klayman challenging the legality of the mass surveillance had “demonstrated a substantial likelihood of success” and he agreed to Klayman’s request for a preliminary injunction suspending the program.
"I cannot imagine a more ‘indiscriminate’ and ‘arbitrary invasion’ than this systematic and high-tech collection and retention of personal data on virtually every single citizen for purposes of querying it and analyzing it without judicial approval," Leon wrote.
"Surely, such a program infringes on 'that degree of privacy' that the Founders enshrined in the Fourth Amendment."
But Leon immediately put his ruling on hold pending an expected appeal by the government.
Klayman's lawsuit, which is aimed at stopping the government from collecting his own so-called metadata, is one of numerous cases filed in US courts since the controversial surveillance program was revealed by former NSA contractor-turned-leaker-turned fugitive Edward Snowden last summer.
In a statement published by The New York Times, Snowden said the ruling vindicated his actions.
“I acted on my belief that the NSA's mass surveillance programs would not withstand a constitutional challenge, and that the American public deserved a chance to see these issues determined by open courts,” Snowden wrote.
“Today, a secret program authorised by a secret court was, when exposed to the light of day, found to violate Americans’ rights. It is the first of many.”
A copy of the ruling was posted on the document sharing website scribd.com.